The present invention relates generally to pump control systems and, more particularly, to closed loop pump output control systems for variable displacement piston pumps.
Variable displacement piston pumps typically have swash plates or trunions within their housings which may be adjusted so as to control the pump output for a given pump speed. These swash plates are often rotatable about an axis, and the position of the swash plate is proportional to the pump output level. To provide precise control of the pump outputs, prior art devices have used the swash plates to create feedback signals which may be compared to signals representing the desired pump output. A qualitative difference or "error" between these signals may be used to create a control signal to various means for adjusting the swash plate position and, thus, the pump output.
Previous control systems have used hydraulic as well as electrical signals and signal comparison means. Hydraulic circuits for this purpose typically include complicated differential pressure comparators and suffer from leakage problems as well as excessive weight. In addition, complex hydraulic circuits are expensive to fabricate and take up considerably more space than corresponding electrical circuits. Electrical circuits for this purpose usually have a much faster response time and greater signal stability. Prior electrical circuits typically create swash plate position feedback signals by means of linear variable differential transformers (LVDTs) attached to the swash plate controlling piston or by means of rotary potentiometers attached to the swash plate exteriorly of the pump housing by means of mechanical linkages. LVDTs are usually spring biased variable inductors and require an a.c. control system. While potentiometers permit a d.c. control system, their connecting mechanical linkages are complicated and often wear down and result in slack which decreases system response time. Since the potentiometers are remote from the movement of the actual swash plate, there are also inherent loses in accuracy. Further, rotatable potentiometers will often wear out faster than LVDTs, since they have sliding contact surfaces.
Accordingly, the need has arisen to provide a simplified electronic control system for variable displacement pumps having increased precision and stability as well as an extended useful life.